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How to apply homography/inverse homography in YUV 420 semi-planar images?

I have an image in YUV 420 semi-planar format where the bytes are stored in such manner:

[Y1 Y2 ...   [U1 V1....
 Yk Yk+1...]  Uk' Uk'+1]

where size of the Y plane is twice that of UV plane and for every 2*2 matrix of Y values, there is 1 U and 1 V value.

I want to apply a homography matrix to this image without converting it to RGB. So, it's easy to do this for Y plane as Y values have a one-to-one mapping with the x-y pixel coordinate of image but how to do this for the UV plane as UV values don't have a direct mapping with the x-y pixel coordinates?

Solution

  • I did it by filling the UV-plane only for even x-y coordinates.

    First, I found out the Y-coordinate in the original image for that pixel location by applying inverse homography. Then, I found the corresponding UV-coordinate for that Y-coordinate and used the value at that location to fill the UV-plane of my new image.